An electro-optic material has at least two “display states,” the states differing in at least one optical property. An electro-optic material may be changed from one state to another by applying an electric field across the material. The optical property may or may not be perceptible to the human eye, and may include optical transmission, reflectance, or luminescence. For example, the optical property may be a perceptible color or shade of gray.
Electro-optic displays include the rotating bichromal member, electrochromic medium, electro-wetting, and particle-based electrophoretic types. Electrophoretic display devices (“EPD”), sometimes referred to as “electronic paper” devices, may employ one of several different types of electro-optic technologies. Particle-based electrophoretic media include a fluid, which may be either a liquid, or a gaseous fluid. Various types of particle-based EPD devices include those using encapsulated electrophoretic, polymer-dispersed electrophoretic, and microcellular media. Another electro-optic display type similar to EPDs is the dielectrophoretic display.
An electro-optic display device may have display pixels or sub-pixels that have multiple stable display states. Display devices in this category are capable of displaying (a) two or more display states, and (b) the display states are considered stable. The display pixels or sub-pixels of a bistable display may have first and second stable display states. The first and second display states differ in at least one optical property, such as a perceptible color or shade of gray. For example, in the first display state, the display pixel may appear black and in the second display state, the display pixel may appear white. The display pixels or sub-pixels of a display device having multiple stable display states may have three or more stable display states, each of the display states differing in at least one optical property, e.g., light, medium, and dark shades of a particular color. For example, the display pixels or sub-pixels may display states corresponding with 4, 8, 16, 32, or 64 different shades of gray.
With respect to capability (b), the display states may be considered to be stable, according to one definition, if the persistence of the display state with respect to display pixel drive time is sufficiently large. An exemplary electro-optic display pixel or sub-pixel may include a layer of electro-optic material situated between a common electrode and a pixel electrode. The display state of the display pixel or sub-pixel may be changed by driving a drive pulse (typically a voltage pulse) on one of the electrodes until the desired appearance is obtained. Alternatively, the display state of a display pixel or sub-pixel may be changed by driving a series of pulses on the electrode. In either case, the display pixel or sub-pixel exhibits a new display state at the conclusion of the drive time. If the new display state persists for at least several times the duration of the drive time, the new display state may be considered stable. Generally, in the art, the display states of display pixels of liquid crystal displays (“LCD”) and CRTs are not considered to be stable, whereas electrophoretic displays, for example, are considered stable.
As compared with an LCD or CRT, a longer time may be required to update an image on an electro-optic display. Accordingly, any reduction in image update time would be desirable. In addition, management of the update process for an electro-optic display may require more host activity than is required with an LCD. Moreover, as electro-optic displays with refresh times faster than past display devices become available, video rendering on an electro-optic displays may become feasible, which will further increase the display update management burden on a host. A host may need to handle more frames in the same time period. Furthermore, color electro-optic displays may become commercially available and management of the update process for color electro-optic displays may require more host activity than that required for gray-scale displays. Accordingly, the capability to update an electro-optic display with minimal host involvement would be desirable.